For obvious safety reasons, important items of equipment in a motor vehicle are fitted with devices for detecting and signalling any operational anomaly.
The on-board generator, usually consisting of an alternator with rectified phase voltages, is one of these important items of equipment, and every driver is familiar with the position of the red light marked “charge” on the dashboard of his car.
The widespread use of electronics on board vehicles, often as a replacement for electromechanical components, enables ever-finer detection of operating faults and of the circumstances of breakdowns, for either diagnostic or communication purposes.
For example, the electronic voltage regulator described in patent FR2642580 contains fault-detecting means which analyse the correspondence between various accessible parameters. In this patent application, in particular, the correspondence is analysed between:                the voltage at the output of the alternator and the voltage at the terminals of the field winding;        the voltage at the output of the alternator and the voltage at the terminals of the phase inputs.        
The second case allows the following faults to be detected indirectly:                no rotation of the alternator (broken fan belt);        an open excitation circuit (open field winding, brush jammed in its housing, etc.).        
According to the teaching of document FR2642580, the following abbreviations are used:
UB: the output voltage of the alternator (or of the battery connected to the alternator).
Un: a desired value.
Up: amplitude of the phase signals.
Vs2: predetermined phase voltage threshold, below which it is assumed that the alternator is unable to charge the on-board network: Vs2<UB
LT: state of the charging light (lamp, LED, LCD, etc.), taking the value 0 if no fault is detected (light off), or the value 1 if a fault is detected (light on).
A charging fault is detected when the voltage UB delivered by the alternator is less than the desired value Un when loads are applied, and the amplitude Up of the phase voltage is less than the threshold Vs2 despite the alternator being set to full excitation, i.e. there is the relationship:If UB<Un and Up<Vs2, then LT=1 (light on).
The method for indirect detection of the failure of the excitation circuit of a polyphase alternator described in the document FR2642580 is, however, not completely satisfactory.
In fact, new alternators often include permanent magnets fixed to the magnetic circuit of the field winding (rotor). These magnets greatly increase the remanent magnetic flux in the alternator.
If there is no excitation current, the phase signals may reach significant amplitudes at high rotation speeds. It is even possible to discharge the alternator without excitation current, which remains acceptable if this discharge remains low enough to be absorbed by the permanent loads present in the vehicle. However, the energy produced by the magnets alone is not sufficient to maintain the voltage in the on-board network when heavy loads are being supplied.
If there is a breakage in the field winding, the drop in amplitude of the phase signals thus does not exist beyond a certain speed of rotation with the magnet rotors.
Consequently: the drop in amplitude of the phase signals cannot be used to detect a continuity fault in the excitation circuit (field winding disconnected, brush jammed in its housing and no longer making proper contact, etc.), because the teaching in document FR2642580 always leads to the relationship:If UB<Un and Up>Vs2, then LT=0 (light off, no charging fault detected).